Demand for small actuators has been increasing in the fields of medical instruments, equipment for industrial use, personal robot, micromachine, and the like.
Polymer actuators have been proposed as one type of small-size actuator. The polymer actuators are roughly classified into two types, i.e., (1) one that makes use of expansion/contraction through reduction/oxidation of an electron conducting polymer such as polypyrrole and polyaniline in an electrolyte (electron-conducting polymer actuator), and (2) one that comprises an ion-exchange membrane and a junction electrode and can function as an actuator by applying an electric potential difference to the ion-exchange membrane in a water-containing state to cause flection or deformation of the ion-exchange membrane (ion-conducting polymer actuator).
Both of the conventional electron-conducting polymer actuators and ion-conducting polymer actuators have been used mainly in an aqueous solution of electrolyte because an electrolyte is necessary for the operation thereof. This severely limits the applications of the polymer actuators. Therefore, development of an actuator element that may be driven in air is essential for allowing the polymer actuators to be practically used in a wider range of applications.
In order to drive the polymer actuators in air, it is necessary to prevent evaporation of water. To make it happen, a method of coating resins has been proposed. However, with this method, it is difficult to coat the resins completely, the coating may be easily broken by a small amount of gas generated from electrode reactions, and the resistance against deformation response increases due to the coating. Therefore, the method has not come into practical use. Additionally, for example, organic solvents having a high boiling point, such as propylene carbonate, have been used instead of water. However, this method has the same problems as described above. Moreover, these solvents have smaller ion conductivity than water, so that a problem of lower responsiveness is caused. Further, another problem in terms of durability is caused because of oxidation/reduction reactions on electrode surfaces.
In order to overcome these problems, actuator elements have been proposed, which can operate in air or in vacuum by using a gel of carbon nanotube and ionic liquid as a conductive expandable active layer. (See Polymer Preprints, Japan, vol. 53, 2nd, 2004, pp. 4816-4817, p. 2355).
These actuator elements, for example, have a structure in which their conductive layers, which include conductive filler, an ionic liquid and a polymer, are adhered to both surfaces of a gel electrolyte film including a polymer and an ionic liquid. When an electric potential difference is applied across the conductive layers, the ionic liquid is polarized, anions move toward a positive electrode and cations move toward a negative electrode. In the ionic liquid conventionally used, cation is bigger than anion, so that the negative electrode is expanded and the positive electrode is contracted. As a result, the actuator element is bent to the positive electrode.
For example, an actuator element has been proposed in Japanese Patent No. 4038685, which includes an electrode layer for the actuator element in which the electrode layer is a gel substance including a carbon nanotube, an ionic liquid, and a polymer.
However, it is still required to develop a polymer actuator element that has a large displacement amount, a high displacement rate, and generates a large force as a result of the displacement.